1. Field of the Invention
The present invention concerns a wide-angle optical system that is capable of viewing areas that cannot be covered directly by the field of view of a conventional imaging device.
The invention relates more particularly to the field of area surveillance.
2. Description of the Prior Art
Most existing imaging devices provide an image of the area under surveillance, which is rectangular, with a width in azimuth and a height in elevation of similar magnitude, for example in a 4/3 ratio for the standard television format. Such an imaging device equipped with an objective of appropriate focal length monitors an area 40.degree..times.30.degree., for example.
It is increasingly necessary to monitor areas having a relatively greater angular width and a relatively smaller angular height, typically 120.degree..times.10.degree., that is to say a relatively wide azimuthal field compared to a relatively narrow field in elevation. For example, a very wide linear field is required in ballistics to identify a few points on the trajectory of the projectile or a mobile, or in security applications to monitor a long fence.
Conventional imaging devices are often of the solid state infrared detector type. The infrared detector comprises a matrix array of sensors each comprising a CCD (Charge-Coupled Device) charge transfer cell. These detectors generally operate in the band of wavelengths from 3 to 5 micrometers or in the band from 8 to 12 micrometers.
A first solution could consist in a new detector with a field covering directly the view to be imaged, typically 120.degree..times.10.degree.. The development of a dedicated detector of this kind would be very costly.
Another solution would be to associate a detector known in itself with an anamorphic optical system. The pixels of the resulting image would then also be anamorphic and the resulting image would not be homogeneous in terms of the horizontal and vertical resolutions.
A third solution would be to juxtapose three standard detectors that would each cover one third of the azimuth field, i.e. 120.degree./3=40.degree.. The cost of this latter solution would be prohibitive.